Nmr rf probe coil exhibiting double resonance

ABSTRACT

NMR probe coils designed to operate at two different frequencies, producing a strong and homogenous magnetic field at both the frequencies. This single coil, placed close to the sample, provides a method to optimize the NMR detection sensitivity of two different channels. In addition, the present invention describes a coil that generates a magnetic field that is parallel to the substrate of the coil as opposed to perpendicular as seen in the prior art. The present invention isolates coils from each other even when placed in close proximity to each other. A method to reduce the presence of electric field within the sample region is also considered. Further, the invention describes a method to adjust the radio-frequency tuning and coupling of the NMR probe coils.

CROSS-REFERENCE TO RELATED APPLICATIONS

This nonprovisional application is a continuation of and claims priorityto U.S. nonprovisional application Ser. No. 14/069,686, entitled “NMR RFProbe Coil Exhibiting Double Resonance”, filed Nov. 1, 2013, now U.S.Pat. No. 8,779,768, which is a continuation-in-part of and claimspriority to U.S. nonprovisional application Ser. No. 13/916,231,entitled “NMR RF Probe Coil Exhibiting Double Resonance”, filed Jun. 12,2013, which claims priority to provisional application No. 61/658,706,entitled “NMR RE Probe Coil Exhibiting Double Resonance”, filed Jun. 12,2012, all of which are incorporated herein by reference in theirentireties.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Grant No.1R01EB009772-01 awarded by National Institute of Health. The governmenthas certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates, generally, to nuclear magnetic resonance (NMR).More specifically, it relates to radiofrequency (RF) transmit-receivecoils.

2. Brief Description of the Related Art

Nearly 50% of all the prescription drugs that are in use today werederived from naturally occurring chemicals, also called naturalproducts. In order to identify natural products, they must be fullycharacterized using spectroscopy techniques such as nuclear magneticresonance (NMR). NMR is unique in its ability to provide preciseinformation about molecular structure and dynamics. Though NMR is quitepowerful, its low sensitivity, however, is a major bottleneck in naturalproduct discovery.

To gain maximum sensitivity, NMR spectrometers are designed to operateat high magnetic field strengths, and consequently the spectrum of NMRsignals is in the radio frequency range. The transmit/receive coils arethe probe coils that stimulate the nuclei and detect the NMR responsefrom the sample. The sensitivity of the probe coils is primarilydependent on two factors the quality factor (Q-factor) of the coil andthe filling factor of the coil. The Q-factor can be improved byconstructing the coil out of low loss/resistance materials, such as hightemperature superconducting (HTS) materials (Brey, W. W., Edison, A. S.,Nast, R. E., Rocca, J. R., Saha, S., and Withers, R. S. (2006) Design,construction, and validation of a 1-mm triple-resonancehigh-temperature-superconducting probe for NMR, J Magn Reson 179,290-293; Brey, W. W., Edison, A. S., Hooker, J., Nast, R. E., Ramaswamy,V., and Withers, R. S. (2012) Design, construction and validation of aHigh-Temperature-Superconducting 13C optimized 1.5-mm cryogenicallycooled NMR probe for natural products and metabolomics, In The 53rdExperimental NMR Conference, Miami, Fla.).

The filling factor can be improved by placing the coil very close to thesample. In multi-channel NMR probes, therefore, the sensitivity of eachchannel decreases as the distance between coils and the sampleincreases. Thus, the sensitivity of the probes can be optimized at aninefficient rate of only one channel at a time, namely, the coil placedclosest to the sample. Furthermore, another drawback of the probes andmethodology of the prior art is that the placement of coils in veryclose proximity to each other causes undesirable interaction betweenthem.

Another factor affecting sensitivity, aside from materials used (e.g.,HTS) and proximity to the sample, is the size of the coil size. Smallercoil sizes tend to lead to more insensitive coils.

An NMR probe coil provides the radiofrequency (RF) magnetic field to thesample, thereby stimulating the nuclear spins, and detects the responseof the nuclear spins. When RF current flows through the windings of theNMR probe coil, an RF magnetic field is produced perpendicular to thedirection of the current. An RF transmit current forced into the coilproduces an RF magnetic field in the sample region, which excites thenuclear spins. Conversely, the RF magnetic field caused by theprecession motion of the nuclear spins induces an RF current in the coilwindings. In the transmit mode, the strength of the magnetic fielddecays away with an increase in the distance from the coil, asdetermined by the Biot-Savart law. By reciprocity, in the receive mode,the strength of the induced current in the coil decays as the distancebetween sample and coil increases. Under these conditions, it isdesirable to design the NMR probe coils to be placed as close aspossible to the sample for purposes of optimizing sensitivity.

Another important factor affecting the performance of the RF probe coilsis the Q-factor of the coil. The Q-factor can be improved by loweringthe resistance and thus the loss in the material of the coil. This maybe achieved by either lowering the temperature of the normal metalcoils, or by using superconducting material. NMR probe coils arecommonly fashioned out of HTS materials. This is achieved by patterningthe HTS on planar dielectric substrates. However, such planar coilsoffer significant constraints to placing the coils very close to thesample.

For NMR excitation and detection of multiple channels, conventional RFprobes utilize one (1) pair of coils for each channel. The pair of coilsthat is placed closest to the sample performs at its optimum to achieveexcellent sensitivity. Each additional channel requires a pair of coilsnested outside all the other channels. As channels are added, eachadditional pair of coils must be placed farther away from the sample,providing lower and lower sensitivity. An NMR field frequency lockchannel typically used for analytical NMR requires its own coil pair inaddition to the others. A typical “triple resonance” NMR probe of thetype commonly used for biomolecular structure experiments requires atotal of four (4) nested pairs of coils, and of these, only the innerpair is optimized for sensitivity since it is closest to the sample.

The prior art has attempted to improve upon NMR RF probe coils. Forexample, U.S. Pat. No. 4,973,908 relates to a surface coil for NMRspectroscopy of humans which utilizes a circular coil and a figure-8 orbutterfly coil that is produces a magnetic field substantiallyperpendicular to the circular coil. The '908 patent applies to a surfacecoil rather than a volume coil, applies to human rather than analyticalNMR spectroscopy, is fabricated from freestanding metal rather thandeposited on a dielectric substrate, and relies on discrete rather thandistributed and integrated capacitive elements to tune to the NMRfrequency.

U.S. Pat. No. 4,816,765 describes a surface coil for MRI of human whichutilizes coplanar coils of different shapes to generate orthogonalmagnetic fields. The coils are intended for quadrature MRI applications.

U.S. Pat. No. 5,565,778 discloses a self-resonant structure known as a“racetrack” which incorporates interdigital capacitors into the NMRcoil.

U.S. Pat. No. 5,594,342 describes dividing the current carrying elementsinto thin strips to avoid distortion of the NMR polarizing field.

U.S. Pat. No. 6,201,392 describes to a number of configurations ofparallel superconductive coils to minimize interaction between coils.Simple rectangular coils are partly overlapped or otherwise disposed tonull their mutual inductance. A parallel LC trap can be incorporatedinto the rectangular coil to reduce interaction with other coils at asingle frequency. However, this prior art does not teach orthogonalmagnetic fields as a means to null the mutual inductance between thecoils. Rather, it teaches parallel magnetic fields over the sampleregion. Parallel magnetic fields have several drawbacks, however. Forexample, coil independence can be achieved only by requiring adjustmentof overlap of coils on a single substrate and/or adjustment of thespacing of coils on independent substrates. Further, the '392 patentdoes not teach the use of fixed coupling loops which utilize variablecapacitors to adjust tuning and matching.

U.S. Pat. No. 7,397,246 relates to methods for combining superconductiveand low-Q coils such that the low-Q coils do not spoil the Q of thesuperconductive coils. The methods involve crossovers in the low-Q coilsto reduce capacitive coupling to the superconductive coils.

U.S. Pat. No. 7,446,534 discloses a method to suppress the electricfield of the NMR coil fringing into the sample.

U.S. Pat. No. 8,089,281 relates to doubly resonant surface coils withthe magnetic fields substantially orthogonal to each other.

However, the foregoing prior art suffers from the one or more of thefollowing disadvantages, despite the increased sensitivity seen inprobes formed of HTS materials, Conventional probe coils have beenunable to replace the industry standard 5-mm triple resonance probe usedin laboratories. Probes that are newly developed tend to be niche probesthat are capable of use in very specific applications. Additionally,there are moving elements with HTS probes that are not used in probesbased on metal wires. These moveable wire loops adversely affect staticmagnetic field homogeneity, which makes initial adjustment difficult andreduces the resolution that can be obtained. The loops also can fail andare difficult to repair. Further, patterning multiple coils close toeach other causes interference that can affect the reproducibility ofresults.

In a conventional NMR probe, metal wire or foil loops surrounding thesample convert a tiny RF magnetic field from the sample into electricalsignals which are detected by the spectrometer. The conversion is not anefficient process because of the resistance of the metal. In an HTSprobe, self-resonant coils are formed of thin-film oxidesuperconductors, such as yttrium barium copper oxide (YBCO), instead ofmetal. This is used as NMR detection coils because of their extremelyhigh quality factors and nearly loss-free qualities in the NMR frequencyrange. The film is available as a coating on polished sapphire wafers.Electrical energy is coupled into and out of these coils by means ofinductive coupling to a wire loop at the end of a coaxial transmissionline. Mechanical adjustment of the position of the wire loop is used toadjust the coupling to match the coil impedance to the characteristicimpedance of the transmission line. A related adjustment of radiofrequency properties is known as tuning. Tuning refers to a shift inresonant frequency of the NMR coil. In HTS, this shift is accomplishedby moving a shorted wire loop so that it intercepts a variable amount offlux from the NMR coil.

The moving loop approach for tuning and matching has some importantdisadvantages. Most importantly, moving a loop and coaxial cable closeto the NMR sample tends to change the uniformity of the polarizingmagnetic field. In NMR, chemical resolution is typically limited by theuniformity of the polarizing field. Great effort is made to adjust thisuniformity in a process called “shimming.” Even if the loop is made fromhigh quality susceptibility-compensated wire, the effect is noticeable.It is, therefore, not possible to adjust the RF coupling (known asmatching) or the tuning without affecting the resolution, requiring atime-consuming step of re-shimming the magnet.

Another drawback of moving loops is basic to the use of moving parts inalmost any device. Moving parts tend to be less reliable than otherapproaches, as there is higher chance of inefficiencies and failure.

Additionally, the number of nested pairs required for a triple resonanceNMR probe places a limit on the sample diameter that can practically beaccommodated. To achieve reasonable sensitivity and field homogeneity,each coil pair must be at least as wide as the gap between the coils.Within a “standard bore” NMR magnet, shim and pulsed field gradientcoil, there is not enough room to nest more than about three (3)channels around a standard 5-mm diameter sample tube. There is verylittle space available in NMR probes, and the need for independent loopsfor the two functions makes the design, construction, adjustment andrepair of HTS NMR probes significantly more difficult and timeconsuming. There is insufficient space to accommodate the coils requiredfor a triple resonance probe that requires four (4) channels and four(4) nested pairs.

Accordingly, what is needed is an NMR probe that has multiple RF coilsin close proximity with each other and with the sample, while producinga strong and homogenous magnetic field at both frequencies that reduceselectric fields within the sample region and while also minimizing theinteraction between the RF coils. With HTS materials, fewersuperconducting materials should be needed, thus allowing for a larger,standard-sized sample, while eliminating moving parts, in turn improvingreliability and reproducibility of results. However, in view of the artconsidered as a whole at the time the present invention was made, it wasnot obvious to those of ordinary skill in the field of this inventionhow the shortcomings of the prior art could be overcome.

All referenced publications are incorporated herein by reference intheir entirety. Furthermore, where a definition or use of a term in areference, which is incorporated by reference herein, is inconsistent orcontrary to the definition of that term provided herein, the definitionof that term provided herein applies and the definition of that term inthe reference does not apply.

While certain aspects of conventional technologies have been discussedto facilitate disclosure of the invention, applicants in no way disclaimthese technical aspects, and it is contemplated that the claimedinvention may encompass one or more of the conventional technicalaspects discussed herein.

The present invention may address one or more of the problems anddeficiencies of the prior art discussed above. However, it iscontemplated that the invention may prove useful in addressing otherproblems and deficiencies in a number of technical areas. Therefore, theclaimed invention should not necessarily be construed as limited toaddressing any of the particular problems or deficiencies discussedherein.

In this specification, where a document, act or item of knowledge isreferred to or discussed, this reference or discussion is not anadmission that the document, act or item of knowledge or any combinationthereof was at the priority date, publicly available, known to thepublic, part of common general knowledge, or otherwise constitutes priorart under the applicable statutory provisions; or is known to berelevant to an attempt to solve any problem with which thisspecification is concerned.

BRIEF SUMMARY OF THE INVENTION

The long-standing but heretofore unfulfilled need for a single coil NMRprobe that can operate at two frequencies, while producing a strong andhomogenous magnetic field at both frequencies that reduces electricfields within the sample region is now met by a new, useful, andnonobvious invention.

In an embodiment, the current invention is an NMR probe. The NMR probeincludes a first RF coil and a second RF coil patterned on a dielectricsubstrate, and a pair of such dielectric substrates with a sample regiontherebetween configured to receive an NMR sample. The first and secondRF coils are patterned on a single dielectric substrate, are formed ofconductive materials, and generate magnetic fields that are eachresonant with an operating frequency that may be the same or different.The magnetic fields of the first and second RF coils are orthogonal toeach other at their respective operating frequencies in the sampleregion. With this structure and configuration, these magnetic fieldsexcite and detect their respective NMR signals at their operatingfrequencies simultaneously.

The first and second RF coils can each include a set of current-carryingelements that are exclusive of each other at their respectiveradiofrequencies/operating frequencies. Alternatively, thecurrent-carrying elements can be the same, such that resonances areproduced from the current-carrying elements by differences in currentdensity distribution at the operating frequencies.

Structurally, the RF coils can be configured in a variety of manners.The RF coils can be patterned on opposite sides of a single dielectricsubstrate. Optionally, a Faraday shield may be positioned between theradiofrequency coils, where said Faraday shield would shield theelectric field generated by said second coil distal to the sample, butwould not affect the magnetic field generated by said first RF coilproximal to the sample. Further, the first and second RF coils can be aspiral coil and a figure-8 racetrack resonator, respectively. In thisembodiment, the spiral coil may generate its magnetic fieldsubstantially perpendicular to the plane of the dielectric substrate,and the figure-8 racetrack resonator may generate its magnetic fieldsubstantially parallel to the plane of the dielectric substrate. Here,the figure-8 racetrack resonator coils on either side of the sample maycarry current in counter directions so that the generated magneticfields are additive and parallel to the plane of the dielectricsubstrate.

Alternatively, the configuration can be such that the dielectricsubstrate is formed of multiple substrates that are fastened together.In this case, the RF coils would be patterned on the two differentdielectric substrates.

Alternatively, the configuration can be such that the RF coils arepatterned on the same side of the dielectric substrate in a manner thatone RF coil is patterned within the other RF coil. Here, the first andsecond RE coils can be a spiral coil and a racetrack resonator,respectively, with the racetrack resonator positioned within the spiralcoil. In this embodiment, the spiral coil may generate its magneticfield substantially perpendicular to the plane of the dielectricsubstrate, and the racetrack resonator may generate its magnetic fieldsubstantially parallel to the plane of the dielectric substrate.

The NMR probe may further include capacitive coupling betweencurrent-carrying elements of each RF coil across the dielectricsubstrate at either or both of the operating frequencies.

The NMR probe may further include inventive RF coils in the shape of afigure-8. The current in the probe at the operating frequencies wouldflow through the central conductor of the figure-8, such that themagnetic fields generated by the coils would be substantially parallelto the plane of the dielectric substrate. Further, the central conductorcan have ends that are tapered (i.e., wider in the middle than at theends) in order to improve homogeneity of the magnetic fields.

The NMR probe may further include a fixed inductive coupling loop fortransferring energy into and out of the RF coils, The coupling loopswould be terminated by a network of capacitors that are adjustable toachieve tuning and coupling of the RF coils. The coupling loops may beformed of non-superconductive metallic conductors (i.e., normal metallicconductors) or high temperature superconductors, among other suitablematerials.

In a separate embodiment, the current invention is an NMR probe. The NMRprobe includes a pair of dielectric substrates each comprising a spiralcoil and a figure-8 racetrack coil, a sample region therebetweenconfigured to receive an NMR sample, capacitive coupling between thecurrent-carrying elements of the coils, and a fixed inductive couplingloop. The RF coils are patterned on a single dielectric substrate, areformed of conductive materials, and generate magnetic fields that areeach resonant with an operating frequency that may be the same ordifferent. The magnetic fields of the RF coils are orthogonal to eachother at their respective operating frequencies in the sample region.The current-carrying elements of the coils are exclusive of each otherat their respective radiofrequencies/operating frequencies.Structurally, the spiral coil is patterned on one side of the dielectricsubstrate, and the figure-8 racetrack resonator is patterned on theopposite side of the dielectric substrate. The figure-8 racetrack coilson either side of the sample carry current in counter directions so thatthe generated magnetic fields are additive and parallel to the plane ofthe dielectric substrate. A Faraday shield is positioned between thespiral RF coils, where said Faraday shield would shield the electricfield generated by the spiral coil distal to the sample, but not affectthe magnetic field generated by said racetrack coil proximal to thesample. As discussed, the NMR probe includes capacitive coupling betweencurrent-carrying elements of each RF coil across the dielectricsubstrate at either or both of the operating frequencies. The figure-8racetrack coil comprises a central conductor patterned along thelongitudinal axes of the RF coils. The current in the figure-8 coil atthe operating frequencies would flow through the central conductor, suchthat the magnetic fields generated by the coils would be substantiallyparallel to the plane of the dielectric substrate. Further, the centralconductor can have ends that are tapered (i.e., wider in the middle thanat the ends) in order to improve homogeneity of the magnetic fields. Thefixed inductive coupling loop is formed of high temperaturesuperconductors and transfers energy into and out of the RF The couplingloops would be terminated by a network of capacitors that are adjustableto achieve tuning and coupling of the RF coils. With this structure andconfiguration, these magnetic fields excite and detect their respectiveNMR signals at their operating frequencies simultaneously.

in a separate embodiment, the current invention is a method ofminimizing interaction between two (2) coils positioned in closeproximity to each other. The two coils are patterned on a singledielectric substrate, and each generates its own magnetic field resonantat an operating frequency. The coils are positioned such that themagnetic fields are orthogonal to each other at their respectiveoperating frequencies. With this methodology, the net magnetic fluxgenerated by one coil and flowing through the other coil would be zero(0).

These and other important objects, advantages, and features of theinvention will become clear as this disclosure proceeds.

The invention accordingly comprises the features of construction,combination of elements, and arrangement of parts that will beexemplified in the disclosure set forth hereinafter and the scope of theinvention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the invention, reference should be made tothe following detailed description, taken in connection with theaccompanying drawings, in which:

FIG. 1A is a perspective view of a coil according the prior art.

FIG. 1B is a perspective view of a single channel HTS probe according tothe prior art

FIG. 1C is a cross-sectional layout of a 4-channel probe showing four(4) pairs of conventional HTS coils, according to the prior art.

FIG. 2A is a rear view of a ¹³C-¹H coil according to an embodiment ofthe current invention.

FIG. 2B is a front view of the ¹³C-¹H coil of FIG. 2A.

FIG. 2C is a cross-sectional layout of a 4-channel probe usingdouble-resonance HTS coils according to the current invention. It can beseen that the number of elements in the probe can be cut in half withthe use of double-resonance coils.

FIG. 3 depicts a ¹⁵N-²H coil according to an embodiment of the currentinvention.

FIG. 4 is a mechanism for coupling electrical energy into and out of theself-resonant RF coils, according to an embodiment of the currentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

in the following detailed description of the preferred embodiments,reference is made to the accompanying drawings, which form a partthereof, and within which are shown by way of illustration specificembodiments by which the invention may be practiced. It is to beunderstood that other embodiments may be utilized and structural changesmay be made without departing from the scope of the invention.

in an embodiment, the current invention is an NMR RF probe coil thatincludes two NMR sample coils on a single substrate to allow bettersensitivity for the second channel. Each RF coil is formed of conductivematerial patterned on a single dielectric substrate. The two sets ofcoils thrm a sample region between them for positioning the NMR sample.The coils can be used in existing NMR RF probes. These probes are usedfor chemical identification and structural analysis of molecules.

Using this NMR RF probe coil, two NMR coils can be placed in proximityto the NMR sample instead of one as in the conventional art. In anembodiment, the NMR RF probe coil includes superconductive oxide coilspatterned on a flat dielectric substrate. Sensitivity and otherperformance aspects are improved for the second coil, which can be usedfor a second nuclear isotope. For example, the two nuclear isotopes canbe and ¹³C analyzed simultaneously. The novel arrangement and coilstructures produce a uniform magnetic field over the sample space andresonate at the required radiofrequencies.

The probe coil is structured reduces the space required for the coils,allowing for larger samples in standard NMR magnets. This structurewould permit function of a standard 4-resonance probe (¹H, ¹³C, , ²H,¹⁵N) for a standard 5-mm diameter sample tube in a standard diametermagnet. Contrastingly, the conventional art has taught that highsensitivity superconductive probes are limited to smaller samples orfewer channels. With two coils on a single substrate, the number ofcoils in each probe is reduced by 50%, reducing complexity and cost.

in a contemplated arrangement of the probe coil, each coil set at one ofthe two resonance frequencies are exclusive of the current-carryingelements at the other frequency. The coils can be placed on thedielectric substrate in a variety of configurations. One configurationincludes the coils placed on opposite sides of one dielectric substrate.Another configuration includes the coils patterned on two separatedielectric substrates that are fastened together. Another configurationincludes the coils placed on the same side of one dielectric substrate,where one coil is placed within the other coil.

In an alternative arrangement of the probe coil, the current-carryingelements are not exclusive of each other at the two resonancefrequencies. Capacitive coupling can be achieved between the two coilsacross the substrate at one or both of the two frequencies.

In an alternative arrangement of the probe coil, the NMR probe cancomprise the current-carrying elements of each coil set at twofrequencies that are the same. Two resonances can then be produced dueto differences in the distribution of current density at the twofrequencies.

In certain embodiments, the current invention further contemplates amethod of using two RF coils in close proximity to one another with verylittle interaction. The two coils are positioned such that the netmagnetic flux generated by one coil and flowing through the other coilis zero (0). As a result of this structure and positioning, the magneticfield produced by one coil at its operating frequency is orthogonal tothe magnetic field produced by the other coil. The NMR probe coilincludes conductive elements patterned on a dielectric substrate for aresonant device. The current in the coil at the resonance frequencyflows through a central conductor, and flows back in the reversedirection through distal conductors. This creates a magnetic fieldwithin a sample region that is parallel to the dielectric substrate.

The central conductor may be wide near the middle of its structure andtapered along the ends along the longitudinal axis of the coil. Thisstructure can help improve the homogeneity of the magnetic field of theNMR probe coil.

A single fixed loop can be used to couple electrical energy into and outof the HTS NMR coils. The loop is terminated by a network of trimmercapacitors that are adjusted to vary both the coupling and the resonantfrequency of the NMR coil. This single fixed loop replaces two loopsused in the prior art.

In order to allow for adjustments to tuning and matching, the singleloop is in an over coupled condition to the NMR coil. This means thatthe impedance at the loop terminals looking toward the coil at the coilresonance frequency is less than the characteristic impedance of thetransmission line. The amount of coupling needed can be predeterminedand preset based on the inductance and quality factor of the NMR coil,the tuning range needed, and the anticipated loss in the NMR sampleitself.

Further, electrical loss should be minimized in the fixed coupling loop.This electrical loss can occur as a function of two processes. First,there may be electrical loss due to the desired currents induced alongthe length of the coupling loop needed for coupling and tuning. Theseloops increase proportionally with the series resistance of the wire. Inthe typical limit for RF circuits, where the wire thickness is muchgreater than the skin depth, the series resistance varies inversely withthe wire radius. This transport loss is then inversely proportional towire radius, so it would be desirable to use a wire of large radius.However, magnetic flux perpendicular to the finite surface area of thewire induces so-called eddy currents in the wire, also contributing toelectrical loss. A wire of larger radius would be subject to greatereddy current loss. As such, there is an optimal wire radius which can bedetermined for each case. The location and shape of the fixed couplingloop can also be adjusted to maximize coupling while minimizing eddycurrent loss. Because the loop is fixed, the wire would remain in theconfiguration of minimum loss at all times.

In certain embodiments, the current invention teaches doubly resonantcoils that generate strong and homogenous magnetic field at tworesonance frequencies. The current distribution in these two resonancemodes is such that the magnetic fields within the sample region areorthogonal to each other. In an embodiment, a set of two coils whosemagnetic fields are orthogonal to each other within the sample region isused to excite and detect the two resonance frequencies, therebyallowing for independent design optimization with almost negligibleinteraction between the two coils. In another embodiment, quadraturedetection of NMR signal at one frequency may be achieved by positioningtwo coils that operate at the same frequency.

Conventional NMR probe coils, such as those seen in FIGS. 1A-1C known inthe prior art. and operating at their fundamental resonance frequency,generate a magnetic field perpendicular to the substrate of the coil.The pair of coils straddling the sample on either side forms a Helmholtzpair, and the magnetic field homogeneity is determined by the Helmholtzcondition. FIG. 1B illustrates the arrangement of a single channel HTSprobe: a pair of self-resonant HTS coils mounted on a coldhead, aninductive loop for coupling the RF energy from the coil, and anotherloop for frequency fine-tuning. Each additional channel requires the useof another set of coils and loops. The most useful configuration of NMRprobe is known as ‘triple resonance’ because it includes channels forthree of the most biologically significant elements: hydrogen, carbonand nitrogen. A fourth channel (deuterium) is included to regulate themagnetic field. An HTS probe of this nature requires the use of fourpairs of coils as shown in FIG. 1C, along with the associated tuning andcoupling loops.

Contrastingly, the current invention is a double-resonance probe coilthat produces a strong and homogenous magnetic field at two frequenciessimultaneously. The double-resonance coils provide optimum NMR detectionsensitivity of both carbon and hydrogen as shown in FIG. 2C. They aresuperior to single-resonance designs because they allow idealsensitivity of two channels simultaneously. Also, they reduce theexpense and the complexity by reducing the number of coil pairsrequired.

In an embodiment, the present invention is an NMR coil that generates amagnetic field parallel to the substrate of the coil. The pair of coilson either side of the sample carries currents in counter directions, inorder that the magnetic fields from both coils are additive. Thehomogeneity of the magnetic field can be optimized by adjusting thewidth and shape of the central conducting strip.

The presence of electric field within the sample region can be a sourceof loss in NMR experiments. In the design of NMR probes, it may bedesirable that the coils have the lowest possible electric field in thesample region, so as to achieve high sensitivity. Various means ofreducing the electric field are known in the art. In an embodiment ofthe invention, the coil used to generate the magnetic field at one ofthe frequencies can be used as a mechanism to shield the electric fieldat the other frequency. In a further embodiment, dedicated electricfield shields are used to reduce the electric field penetrating thesample region.

It will be appreciated by those skilled in the art that a number ofvariations are possible within the spirit and scope of the invention.The scope of the invention should not be limited by the specificexamples given, but by the appended claims.

Example 1

FIGS. 2A and 2B illustrate an embodiment of the current invention, a¹³C-¹H coil generally denoted by the reference numeral 11, where coilstructures 10, 14 are placed on opposite sides of one dielectricsubstrate. Exemplary coil 11 would be appropriate to use as the innercoil pair in an NMR probe designed for detection of both ¹³C and ¹Hisotopes. In this embodiment, a dielectric substrate (not shown)separates two superconductive films patterned into self-resonant coilstructures 10, 14. Two such films are disposed around a cylindricalsample as in the prior art to produce a uniform RF magnetic field acrossthe sample. The long axis of coil 11 would be oriented along the fieldaxis of the solenoidal NMR magnet.

The aspect of coil 11 distal to the sample can be seen in FIG. 2A and ispatterned into spiral coil structure 10. Spiral coil structure 10produces a field that is substantially perpendicular to the plane of thedielectric substrate. Spiral coil structure 10 is well suited toachieving low resonance frequencies associated with ¹¹C, ¹⁵N and othernuclei, excluding ¹H and ¹⁹F. However, the electric field of spiral coilstructure 10 fringes away from the dielectric substrate and into thesample under analysis. The conductivity and dielectric loss of thesample are often enough to reduce the Q-factor of the coil and tocontribute to the noise of the NMR measurement.

Thus, to improve sensitivity on the ¹³C channel, coil 11 includesFaraday shield 12 on the aspect of coil 11 proximal to the sample asdescribed in U.S. Pat. No. 7,446,534, which is incorporated herein byreference. Shield 12 includes thin, closely spaced wires that do notgreatly affect the magnetic field produced by spiral coil structure 10.The high frequency (typically ¹H) resonator is patterned on the “front”side of each dielectric substrate, facing the sample, as seen in FIG.2B.

The building block for the ¹H coil is the “racetrack” resonator asdescribed in U.S. Pat. No. 5,565,778, which is incorporated herein byreference. Two racetrack resonators 14 are patterned adjacent to eachother. The resulting structure resembles the figure-8 coil described inU.S. Pat. No. 4,973,908, which also is incorporated herein by reference.Racetrack resonators 14 produce an RF magnetic field that issubstantially parallel to the dielectric substrate and orthogonal to theelectric field produced by spiral coil structure 10 on the rear side ofthe substrate.

Racetrack resonator 14 can be readily tuned to the higher frequency ofthe ¹H isotope. When broken with several gaps 16, in this case with four(4) gaps, racetrack. resonator 14 has a low fringing electric field andis suitable for use close to a biomolecular sample. Both spiral coilstructure 10 and racetrack resonator 14 should be patterned into thinparallel wires as taught in U.S. Pat. No. 5,565,778 patent to reducedistortions of the polarizing magnetic field. Therefore, in areas where.spiral coil structure 10 and gaps 16 overlap, where it is not possibleto continue Faraday shield 12, racetrack resonator 14 itself serves as aFaraday shield for spiral coil structure 10 and does not greatly affectthe magnetic field of spiral coil structure 10.

In NMR spectroscopy, it is important to produce a uniform RE magneticfield over the sample. The field of the figure-8 coil formed by adjacentracetrack resonators 14 is not as uniform, in general, as that of thepair of rectangular resonators. However, the uniformity can be improvedby widening the central region of center portion 18 of the figure-8coil. it may be advantageous for RE homogeneity to taper center portion18 at the ends as shown in FIG. 2B.

Example 2

FIG. 3 illustrates another embodiment of the current invention, a ¹⁵N-²Hcoil generally denoted by the reference numeral 21, where the coilstructures 22, 24 are positioned on the same side of one dielectricsubstrate and one coil is placed within the other coil. Exemplary coil21 would be appropriate to use as the outer coil pair in an NMR probedesigned for decoupling on the ¹⁵N channel and for engaging a ²H fieldfrequency lock. in this embodiment, both self-resonant coil structures22, 24 are patterned on the same side of the dielectric substrate,thereby eliminating the two-sided patterning of the HTS coils.

The longitudinal axis of coil 21 would he oriented along the field axisof the solenoidal NMR magnet. Coil 21 includes figure-8 coil structure22 tuned to the ²H frequency and spiral coil structure 24 tuned to the¹⁵N frequency. The magnetic field in the sample region at the spiralcoil structure resonance frequency is substantially perpendicular to thedielectric substrate, whereas the magnetic field in the sample region atthe resonance frequency of coil structure 22 is substantially parallelto the dielectric substrate. The central region of the center portion 18of figure-8 coil structure 22 is widened to provide better RFhomogeneity.

FIG. 4 illustrates a mechanism according to an embodiment of the currentinvention for coupling electrical energy into and out of theself-resonant RF coils or HTS NMR coils.

fixed loop 30 is positioned in the probe such that it is inductivelycoupled to the RF coils. Loop 30 would be positioned in proximity to theresonant coil. Single fixed loop 30 replaces the moving tuning andcoupling loops for a coil pair in the conventional art. Thus, a networkof trimmer capacitors 32, 34 are included to terminate loop 30. Trimmercapacitors 32, 34 are adjusted to vary both the coupling and theresonant frequency of the NMR coil. Adjusting trimmer capacitors 32, 34does not affect the resolution of the probe, requiring re-shimming eachtime the tuning and coupling are adjusted. The parallel capacitor 32 canbe varied to tune the frequency of the self-resonant coil. The seriescapacitor 34 can be adjusted to match the coil impedance to theimpedance of the transmission line 36. Tuning rods (not seen) can beadded to access the variable capacitors.

In order to allow for adjustments to tuning and matching, single loop 30is in an over coupled condition to the NMR coil. This means that theimpedance at the terminals of loop 30 looking toward the coil at thecoil resonance frequency is less than the characteristic impedance ofthe transmission line. The amount of coupling needed can bepredetermined and preset based on the inductance and quality factor ofthe NMR coil, the tuning range needed, and the anticipated loss in theNMR sample itself.

Further, electrical loss should be minimized in fixed coupling loop 30.This electrical loss can occur as a function of two processes. First,there may be electrical loss due to the desired currents induced alongthe length of coupling loop 30 needed for coupling and tuning. Theseloops increase proportionally with the series resistance of the wire. Inthe typical limit for RF circuits, where the wire thickness is muchgreater than the skin depth, the series resistance varies inversely withthe wire radius. This transport loss is then inversely proportional towire radius, so it would be desirable to use a wire of large radius.However, magnetic flux perpendicular to the finite surface area of thewire induces so-called eddy currents in the wire, also contributing toelectrical loss. A wire of larger radius would be subject to greatereddy current loss. As such, there is an optimal wire radius which can bedetermined for each case. The location and shape of the fixed couplingloop can also be adjusted to maximize coupling while minimizing eddycurrent loss. Because loop 30 is fixed, the wire would remain in theconfiguration of minimum loss at all times,

All referenced publications are incorporated herein by reference intheir entirety, Furthermore, where a definition or use of a term in areference, which is incorporated by reference herein, is inconsistent orcontrary to the definition of that term provided herein, the definitionof that term provided herein applies and the definition of that term inthe reference does not apply.

The advantages set forth above, and those made apparent from theforegoing description, are efficiently attained. Since certain changesmay be made in the above construction without departing from the scopeof the invention, it is intended that all matters contained in theforegoing description or shown in the accompanying drawings shall beinterpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention that, as amatter of language, might be said to fall therebetween.

Glossary of Claim Terms

Capacitance: This term is used herein to refer to the ability of a bodyto store an electrical charge

Capacitive coupling: This term is used herein to refer to the transferof energy within an electric network by means of capacitance betweencircuit nodes.

Central conductor: This term is used herein to refer to an object orsubstance that allows heat, electricity, light or sound to pass along itor through it. Central means it is in or towards the center of the body.

Current-carrying elements: This term is used herein to refer to anaspect of an NMR RF coil that is structured for the flow of a current.

Dieletric substrate: This term is used herein to refer to electricalinsulators, such as silicon, ceramic quartz, etc. It is selected withdielectric strength, dielectric constant and loss tailored for specificcircuit application in order to serve as a base for another material.Generally, it is a nonconductor of electricity with electricalconductivity of less than a millionth (10⁻⁶) of a siemens.

Magnetic flux: This term is used herein to refer to the component of themagnetic B field that passes through a surface. A lower magnetic fluxcorresponds to a lower interaction between magnetic fields generated byseparate coils (i.e., the magnetic field generated by a coil is notpassing through the surface of another coil).

Nuclear magnetic resonance: This term is used herein to refer to aphysical phenomenon in which magnetic nuclei in a magnetic field absorband re-emit electromagnetic radiation. The energy that is re-emitted isat a specific resonance frequency which depends on the strength of themagnetic field and magnetic properties of the isotope of the atoms.

Nuclear magnetic resonance probe: This term is used herein to refer tothe portion of an NMR spectrometer responsible for a significant portionof the work. The probe is placed in the center of the magnetic field,and the sample is inserted into the center of the probe. The probecontains radiofrequency coils (RF) tuned at specific frequencies forspecific nuclei.

Orthogonal: This term is used herein to refer to objects beingperpendicular, non-overlapping, varying independently, or uncorrelated.

Parallel: This term is used herein to refer to two or more straightcoplanar lines that do not intersect.

Perpendicular: This term is used herein to refer to two structures oraspects intersecting or forming a 90 degree (right) angle.

Radiofrequency coil: This term is used herein to refer to coilscontained within the probe tuned at specific frequencies for specificnuclei.

Resonance: This term is used herein to refer to the tendency of a systemto oscillate at varying amplitude at some frequency. The level ofamplitude is greater at some frequencies than others.

What is claimed is:
 1. A nuclear magnetic resonance probe, comprising: afirst radiofrequency coil patterned on a planar dielectric substrate,said first radiofrequency coil formed of conductive material, said firstradiofrequency coil generating a first magnetic field that is resonantat a first radiofrequency; a second radiofrequency coil patterned onsaid dielectric substrate, said second radiofrequency coil formed ofconductive material, said second radiofrequency coil generating a secondmagnetic field that is resonant at a second radiofrequency, said firstradiofrequency coil and said second radiofrequency coil forming a firstdouble-resonance apparatus; a second double-resonance apparatuspositioned substantially parallel to said first double-resonanceapparatus; and a sample region formed between said firstdouble-resonance apparatus and said second double-resonance apparatus,said sample region configured to receive a nuclear magnetic resonancesample, wherein said first magnetic field and said second magnetic fieldexcite or detect respective nuclear magnetic resonance signals at saidfirst radiofrequency and said second radiofrequency alternately orsimultaneously wherein said second magnetic field is orthogonal to saidfirst magnetic field at their respective resonant frequencies in saidsample region.
 2. A nuclear magnetic resonance probe as in claim 1,further comprising: said first radiofrequency coil including a first setof current-carrying elements and said second radiofrequency coilincluding a second set of current-carrying elements, said first set ofcurrent-carrying elements set at said first radiofrequency beingexclusive of said second set of current-carrying elements set at saidsecond radiofrequency.
 3. A nuclear magnetic resonance probe as in claim1, further comprising: said first radiofrequency coil patterned on aside of said dielectric substrate that is proximal to said nuclearmagnetic resonance sample, and said second radiofrequency coil patternedon an opposite side of said dielectric substrate that is distal to saidnuclear magnetic resonance sample.
 4. A nuclear magnetic resonance probeas in claim 3, further comprising: said first radiofrequency coil beinga figure-eight racetrack resonator on a first side of said dielectricsubstrate and said second radiofrequency coil being a spiral resonatoron a second side of said dielectric substrate.
 5. A nuclear magneticresonance probe as in claim 4, further comprising: said figure-eightracetrack resonator generating said first magnetic field substantiallyparallel to the plane of said dielectric substrate, and said spiralresonator generating said second magnetic field substantiallyperpendicular to the plane of said dielectric substrate.
 6. A nuclearmagnetic resonance probe as in claim 3, further comprising: a Faradayshield positioned between said first and second radiofrequency coils,wherein said Faraday shield reduces the electric field generated by saidsecond radiofrequency coil but does not affect said first magnetic fieldgenerated by said first radiofrequency coil.
 7. A nuclear magneticresonance probe as in claim 1, further comprising: said dielectricsubstrate formed of two (2) substrates that are fastened together, andsaid first and second radiofrequency coils patterned on said two (2)substrates, respectively.
 8. A nuclear magnetic resonance probe as inclaim 1, further comprising: said first and second radiofrequency coilspatterned on a single side of said dielectric substrate, wherein saidfirst radiofrequency coil is positioned within said secondradiofrequency coil.
 9. A nuclear magnetic resonance probe as in claim8, further comprising: said first radiofrequency coil being afigure-eight spiral resonator and said second radiofrequency coil beinga spiral resonator.
 10. A nuclear magnetic resonance probe as in claim9, further comprising: said figure-eight spiral resonator generatingsaid first magnetic field substantially parallel to the plane of saiddielectric substrate, and said spiral resonator generating said secondmagnetic field substantially perpendicular to the plane of saiddielectric substrate.
 11. A nuclear magnetic resonance probe as in claim1, further comprising: said first radiofrequency coil or said secondradiofrequency coil being substantially in the shape of a figure-eight,such that a central conductor is patterned along a substantial center ofsaid first or second radiofrequency coils and along a longitudinal axisof said first or second radiofrequency wherein current in said probe atsaid first or second radiofrequencies flows through said centralconductor, wherein said first or second radiofrequency coil carriescurrent in counter directions on opposite sides of said nuclear magneticresonance sample, such that said first and second magnetic fields insaid sample region are additive and are generated substantially parallelto said dielectric substrate.
 12. A nuclear magnetic resonance probe asin claim 11, further comprising: said central conductor having two (2)end portions along said longitudinal axis of said first or secondradiofrequency coil, said two (2) end portions defining a middleportion, said middle portion having a predetermined width, said two (2)ends being tapered relative to said middle portion in order to improvehomogeneity of said first and second magnetic fields.
 13. A nuclearmagnetic resonance probe as in claim 1, further comprising: a fixedinductive coupling loop used to transfer energy into and out of saidfirst and second radiofrequency coils, said coupling loop terminated bya network of adjustable capacitors that are adjustable to achieve tuningand coupling of said first and second radiofrequency coils.
 14. Anuclear magnetic resonance probe as in claim 13, further comprising:said coupling loop formed of high temperature superconductors.
 15. Amethod of minimizing interaction between two (2) coils positioned inclose proximity to each other, comprising the steps of: patterning afirst coil on a planar dielectric substrate, said first coil generatinga first magnetic field resonant at a first frequency, said firstmagnetic field being substantially perpendicular to the plane of saiddielectric substrate, patterning a second coil on said dielectricsubstrate, said second coil generating a second magnetic field resonantat a second frequency, said second magnetic field being substantiallyparallel to said plane of said dielectric substrate, said first andsecond radiofrequency coils defining a nuclear magnetic resonance probecoil; wherein a net magnetic flux generated by said first coil andflowing through said second coil is substantially zero (0).
 16. A methodas in claim 15, further comprising the step of: patterning a centralconductor in said first radiofrequency coil or said secondradiofrequency coil, such that a current in said probe coil at saidfirst or second frequency flows through said central conductor andgenerates said first or second magnetic fields, respectively, parallelto the plane of said dielectric substrate.
 17. A nuclear magneticresonance probe, comprising: a first radiofrequency coil patterned on afirst dielectric substrate, said first radiofrequency coil formed ofconductive material, said first radiofrequency coil generating amagnetic field that is resonant at a radiofrequency; a secondradiofrequency coil patterned on a second dielectric substrate, saidsecond radiofrequency coil formed of conductive material, said secondradiofrequency coil generating said magnetic field that is resonant atsaid radiofrequency; a sample region formed between said first andsecond dielectric substrates, said sample region configured to receive anuclear magnetic resonance sample, wherein said magnetic field excitesor detects nuclear magnetic resonance signals at said radiofrequency anda fixed inductive coupling loop used to transfer energy into and out ofsaid first and second radiofrequency coils, said coupling loopterminated by a network of adjustable capacitors that are adjustable toachieve tuning and coupling of said first and second radiofrequencycoils.
 18. A nuclear magnetic resonance probe as in claim 17, furthercomprising: said coupling loop formed of high temperaturesuperconductors.